Salt bath for annealing metal articles



Patented May 2, 1933 UNITED STATES PATENT OFFICE FAIR-LENDER, OFIFORZHEIM, GERMANY SALT BATH FOR ANNEALING METAL ARTICLES 1T0 Drawing.Application filed June 28, 1930, Serial No. 463,373, and in GermanyJanuary 30, 1930.

The salt baths, serving for annealingarticles made of precious metalsand their alloys, should have the following properties:

The melting point should as much as pos- 5 sible not exceed 600 0., inorder for example to prevent baking together of the thin silversheeting, when it is annealed in roll shape.

The bath liquor must further possess good covering power, that is theannealed article must be covered on withdrawal with a thin and at thesame time dense protecting layer, so that access of air is excluded onthe way to the cooling liquid into which the article annealed is to bedipped, and this article cannot tarnish.

Finally the bath liquor must not act chemically on the articles beingannealed, as otherwise discoloring of the surface and the like willoccur, which is of great disadvantage', especially in the case of plated3.1? ticles. v

The salt baths hitherto known do not possess these properties, and theycannot be used without disadvantage for annealingarticles made ofprecious metals and of the alloys of the same.

It has definitely the necessary thin liquidity necessary for an- 0nealing such articlesby a ding cyanide compounds, but the use of cyanidecompounds is open to the following objections:

They have a great tendency to decompose,

so that the composition of the bath does not remain constant. Thetemperature of the bathchanges with the alteration of the potassium, forexample of cyanide content,if

the bath has to be maintained at a uniform degree of thin liquidity.Moreover, owing to the presence of cyanide compounds vapors are producedwhen quenching the ar-, ticles, which have a great tendency to causesneez' There are also difliculties in get-. tingri of the waste watercontaining cyanide of potassium. H

been possible, attain The salt bathmixtures with a high percentage ofcommon salt are certainly .remarkable for their cheapness, but theypossess very little or no covering power. If they contain soda orpotash, they give for example'a greenish color to plated articles.

' It has been found, that the discolorings and the other objections tothe known salt baths do no longer occur, if to the mixture of commonsalt and potash, to which larger quantities of potassium sulphate areadded, the proportion of soda to the potassium is not greater than 1. Ifthe proportion is inverse, the melting point of the mixture rises in avery short time, that is, if the temperature of the bath is maintainedconstant, it will quickly become viscous and useless.

Suitable compositions for the bath are:

. Percent I. Common salt 46 Potassium sulphate 43 Potassium carbonate"11 For improving the covering power of I Percent II. Common salt 30Potassium sulphate 44 Potassium carbonate 21 BOI'flX 5 0 III. Commonsalt ,.;i 30 Potassium sulphate 44- Potassium carbonate 21 Sodiumphosphates; 5

Baths having a very good covering power, which are not affected by thematerial being annealed and are therefore particularlysuitable forplated articles, are obtained from the above mixtures by the admixtureof mineral alkaline carbonates or oxides up to The following areexamples:

Percent 5 IV. Common salt 28 Potassium sul hate 41. 5 Potassium caronate 21 Borax 4. 5

Magnesium carbonate 2. 5

m Calcium carbonate 2. 5

V. Common salt 25 Potassium sulphate 54 Potassium carbonate Magnesiumcarbonate 3 15 Calcium carbonate 3 VI. Common salt 25 Potassium sulphate54 Potassium carbonate 15 Magnesium oxide 3 Calcium oxide 3 Theproportion of pure sodium to pure potassium is in the Examples I toVI:1: 1.33; 112.55; 122.5; 1:1.22;1:3.2 and 123.2,

that is not greater than 1.

It may happen, that a bath of this kind becomes black, especially whenheated from above, if this is carried out for protecting the crucible,owing to the flame coming directly into contact with the salt mixture.

In order to overcome this objection 0.1 to 0.2% of saltpeter may beadded to the bath.

The annealing baths are suitable also for many nonprecious metals andalloys.

I claim l. A salt bath for annealing metal articles, consisting in amixture of common salt, potash and potassium sulphate, the proportion ofthesodium to the potassium being not greater than 1, with addition ofmineral alkaline oxides including magnesium to a maximum of 10%.

2. A salt bath as specified in claim 1, comprising in admixture 25%common salt,

5 1% potassium sulphate, 15% potassium carbonate, 3% magnesium oxide and3% calcium oxide. Y I In testimony whereof I afiix my signature.

ERWIN FAHRLANDER.

